1 Answer
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There are three different options for setting colours of specific elements: draw, fill, and text. The colour specified using draw applies to outlines, fill applies to areas (and filled arrow tips, like stealth and latex, but unlike the default), text applies to the text in nodes. color is a shorthand for setting all three of these simultaneously. Just specifying a colour on its own is a shorthand for the shorthand: It also applies to draw, fill and text.

green and color=green are equivalent, and since you're not using text in your example, fill=green, draw=green is also equivalent here.